It is appropriate to recall in this connection that .alpha.-amino acids, especially the so-called essential .alpha.-amino acids, are assuming an indisputable and ever-increasing industrial value. Some of them can in fact be used in human or animal medicine, in particular to mitigate certain dietary deficiencies in animal proteins in developing countries, or to supplement the plant proteins used for feeding cattle or poultry. Others of these compounds can also take part, for example, in the composition of soaps or cosmetics.
In the prior art, .alpha.-amino acids are prepared from their corresponding aldehyde precursors, by the Strecker reaction or by one of the many modifications applied to the original reaction, and in particular by implementing the process described in French Pat. No. 2,372,797. According to the process of this earlier patent, the intermediate .alpha.-amino-nitrile or one of its salts is catalytically hydrolyzed in basic medium, by reacting an aqueous solution containing at least one carbonyl derivative with the said .alpha.-amino-nitrile or one of its salts, in the presence of hydroxide ions. Such a process, which is extremely selective and favorable from an economic standpoint, nevertheless requires the separation of the carbonyl catalyst from the reaction medium and possible recycling of the catalyst. Furthermore, in this earlier process, the choice of carbonyl catalyst is greatly limited by the need for it to be stable in homogeneous basic medium.
The French Patent Application published under No. 2,519,973 describes a process for preparing .alpha.-amino acids which makes it possible to avoid in part the disadvantages pointed out above. In effect, according to this earlier process, the chemical catalytic hydrolysis of the starting .alpha.-amino-nitrile is no longer performed in homogeneous phase but in heterogeneous phase. To this end, a carbonyl catalyst is used which is no longer in solution in the reaction medium, but is a suitable polymeric resin containing carbonyl groups. According to this eariler process, it is observed that, by introducing an .alpha.-amino-nitrile or one of its salts into an aqueous solution containing a suspension of an insoluble carbonyl-containing polymeric resin and hydroxide ions, the .alpha.-amino-amide corresponding to the starting .alpha.-amino-nitrile is obtained very rapidly. The carbonyl-containing resin insoluble in aqueous basic medium can then be separated from the reaction medium by simple filtration or centrifugation, and then be recycled without the need for a regeneration process. Furthermore, instead of the reaction being performed discontinuously, with recycling of the catalyst, the use of a carbonyl-containing polymeric resin enables the process to be carried out continuously by contact of a solution of .alpha.-amino-nitrile or one of its salts with the immobilized carbonyl catalyst.
However, the implementation of such a process for preparing .alpha.-amino acids by continuous chemical catalytic hydrolysis proves to be much more awkward than it appears at first sight. In particular, the gradual decrease in the catalytic activity of the resin is observed, due to poisoning of the terminal carbonyl groups. This substantial decrease in the catalytic activity considerably limits the industrial applications of such a process, and constitutes a major obstacle to the generalized use of the process in the synthesis of .alpha.-amino acids, the international market for which is at the height of expansion.